Small, typically submicron size, particles are formulated into synthetic and natural rubber compounds used for a wide variety of rubber goods such as tires, hoses, belts, gaskets, bushings, etc. A wide variety of particles have been used or proposed for rubber compounding, but the most common is carbon black (CB). These particles allow the material properties of the compound to be substantially designed and improved for the application performance. For instance, they enable control of the rubber stiffness, hardness, modulus, and failure properties. Rubber compounded with a reinforcing CB can have a dramatic improvement in wear resistance and render rubber useful for tire treads and other demanding service applications.
A side effect of formulating rubber with reinforcing particles is that the rubber changes from highly elastic to viscoelastic in nature and the filled rubber dissipates energy when the rubber is mechanically cycled. An important practical consequence of this viscoelastic behavior is that tires dissipate mechanical energy as they flex upon rotation resulting in reduced vehicle fuel economy. Precipitated silica (PS) is commonly used in combination with synthetic rubber for automobile tire treads; the PS provides a rubber compound with somewhat reduced wear, compared with a similar CB based rubber compound, but an attractive improvement in energy loss and therefore tire rolling resistance and vehicle fuel economy.
Generally, CB exists in the form of aggregates, which, in turn, are formed of CB primary particles. In most cases, primary particles do not exist independently of the CB aggregate. While the primary particles can have a mean primary particle diameter within the range of from about 10 nanometers (nm) to about 50 nm, e.g., from about 10 nm to about 15 nm; from about 10 nm to about 20 nm; from about 10 nm to about 25 nm; from about 10 nm to about 30 nm; or from about 10 nm to about 40 nm, the aggregates can be considerably larger. CB aggregates have fractal geometries and are often referred in the art as CB “particles” (not to be confused with the “primary particles” discussed above).
Many types of CB are produced in a furnace-type reactor by pyrolyzing a hydrocarbon feedstock (FS) with hot combustion gases to produce combustion products containing particulate CB. Properties of a given CB typically depend upon the conditions of manufacture and may be altered, e.g., by changes in temperature, pressure, FS, residence time, quench temperature, throughput, and other parameters.
Equipment and techniques for producing CB are known in the art. An example is provided in US RE 28974, a reissue of U.S. Pat. No. 3,619,140, to Morgan et al., both documents being incorporated herein by reference in their entirety. The process involves generating a very hot combustion gas stream moving at very high speed in essentially plug flow by burning a fuel gas such as natural gas with oxygen, in a compact combustion zone and under conditions of very high heat release. Individual streams of liquid hydrocarbon (preheated carbon-black make oil or FS) are injected in a transverse direction to the high-speed combustion stream under conditions by which the liquid hydrocarbon enters the high-speed combustion stream at a linear velocity of more than about 100 feet per second.
The fuel in the combustion zone is completely burned with excess oxygen. CB nuclei are produced once the CB FS is injected and then these nuclei both coalesce and grow into the product CB aggregates.
Techniques for in-situ preparation of silicon-treated CB from CB FS and silicon precursor materials in a CB reactor are disclosed in U.S. Pat. No. 5,904,762 to Mahmud et al.; and U.S. Pat. No. 5,830,930 to Mahmud et al. Further, U.S. Pat. No. 5,830,930 discloses elastomeric compounds incorporating silicon-treated CB. U.S. Pat. No. 6,057,387 to Mahmud et al. discloses aggregate particles comprising a carbon phase and a silicon-containing species phase having certain particle surface area and size distribution characteristics. In such silicon-treated CB, a silicon containing species such as an oxide or carbide of silicon, is distributed through at least a portion of the CB aggregate as an intrinsic part of the CB. Such CB aggregates may be modified by depositing silicon-containing species, such as silica, on at least a portion of the surface of the CB aggregates during formation of the CB aggregates in a CB reactor. The result may be described as silicon-coated CB. In silicon-treated CB, the aggregates contain two phases. One phase is carbon, present as graphitic crystallite and/or amorphous carbon, while the second, discontinuous phase is silica (and possibly other silicon-containing species). The silicon-containing phase may be present in amounts of 0.1 to 25 wt % of the CB aggregate. Thus, the silicon-containing species phase of the silicon-treated CB is an intrinsic part of the aggregate; it is distributed throughout at least a portion of the aggregate or on the surface of the aggregate. U.S. Pat. No. 6,017,980 to Wang et al. discloses elastomer composites comprising aggregates of a carbon phase and 0.1 to 25 wt % of a metal-containing species phase (e.g., Al or Zn) and the formation of such aggregates in-situ in a CB reactor. As an option, a silicon-containing phase may be incorporated with the metal-containing species phase in the CB phase.
U.S. Pat. No. 2,632,713 to Krejci discloses an in-situ treated CB material comprising 0.01 to 10 wt % of a silicon, boron or germanium species. The additive material is introduced to a CB reactor with FS, or separately, and may be added further downstream in the reactor to yield a surface coating on CB particles. CB materials comprising surface domains of silica are disclosed in U.S. Pat. No. 7,351,763 to Linster et al. and in U.S. Pat. No. 6,071,995 to Labauze.
U.S. Pat. No. 6,099,818 issued to Freund et al. describes a process wherein CB nuclei are formed by the partial burning of fuel oil in oxygen-containing gas in the combustion chamber. The CB nuclei are carried by the stream of hot combustion gas into the reaction zone and are immediately brought into contact with the CB raw material forming CB particles that coalesce and grow into aggregates. According to U.S. Pat. No. 6,056,933 issued to Vogler et al., inversion CBs are manufactured in conventional CB reactors by controlling the combustion in the combustion chamber to form CB nuclei which are immediately brought into contact with the CB raw material. U.S. Pat. No. 6,391,274 to Vogler et al., describes a process in which CB seeds (or nuclei) formed in the combustion zone are carried with the flow of combustion gas into the reaction zone where they initiate a seed-induced CB formation with added CB raw material. Silicon-containing compounds such as silanes or silicone oils are mixed with the CB raw material to produce a CB containing 0.01 to 20 wt ° % silicon.
Plasma-based techniques for preparing CB also have been developed. The Kværner process or the Kværner CB & hydrogen process (CB&H), for example, is a method for producing CB and hydrogen gas from hydrocarbons such as methane, natural gas and biogas. According to U.S. Pat. No. 5,527,518, issued to Lynum et al. on Jun. 18, 1996 and incorporated herein by reference in its entirety, a method for producing a carbon black material includes a first stage delivering feedstock through a feed tube to a plasma torch to a reaction area to raise the temperature of the feedstock to about 1600° C., then passing the dehydrogenated carbon material to a second stage to complete the decomposition to carbon black and hydrogen. Additional raw material causes quenching and reaction with formed carbon black to increase particle size density and quantity produced.
U.S. Patent Application Publication No. 2008/0289494 A1 to Boutot et al., published on Nov. 27, 2008 and incorporated herein by reference in its entirety, describes a method and apparatus for a cold arc discharge (CAD) used to decompose natural gas or methane into its gaseous constituents (hydrogen and acetylene) and carbon particles.
According to U.S. Pat. No. 7,452,514 B2, issued to Fabry et al. on Nov. 18, 2008, and U.S. Patent Application Publication No. 2009/0142250 A1 to Fabry et al., published on Jun. 4, 2009 and incorporated herein by reference in their entirety, CB or carbon containing compounds are formed by converting a carbon containing FS, using a process that includes the following steps: generating a plasma gas with electrical energy, guiding the plasma gas through a venturi, whose diameter is narrowing in the direction of the plasma gas flow, guiding the plasma gas into a reaction area, in which under the prevailing flow conditions generated by aerodynamic and electromagnetic forces, there is no significant recirculation of FS into the plasma gas in the reaction area, recovering the reaction products from the reaction area and separating CB or carbon containing compounds from the other reaction products.
In the process described in U.S. Pat. No. 4,101,639, issued on Jul. 18, 1978 to Surovikin et al. and incorporated herein by reference in its entirety, a hydrocarbon FS is introduced in a reaction chamber and into a plasma stream saturated with water vapor.
U.S. Patent Application Publication No. 2015/0210856 to Johnson et al., published on Jul. 30, 2015 and incorporated herein by reference in its entirety, describes a method and apparatus in which a plasma gas is flowed into a plasma forming region having at least one magnetically isolated plasma torch containing at least one electrode. Plasma is collected in a cooled header and flowed to a CB forming region which receives CB forming FS. A gas throat assembly connecting the plasma and the CB forming regions is described by Hoermann et al. in U.S. Patent Application Publication No. 2015/0210858, published on Jul. 30, 2015 and incorporated herein by reference in its entirety.
U.S. Patent Application Publication No. 2015/0210857 A1 to Johnson et al., published on Jul. 30, 2015 and incorporated herein by reference in its entirety, describes combusting FS (typically methane) with plasma in an apparatus having a series of unit operations with individual capacities. The individual capacities of the unit operations are substantially balanced by replacing at least part of the FS with a FS having a molecular weight heavier than methane.
Since a significant quantity of CB material is used to reinforce the rubber components of tires, used tires and other CB reinforced rubber products represent a significant waste stream. To dispose of such waste, used tires can be pyrolyzed and attempts have been undertaken to recover and re-use the carbon-based component.
Generally, pyrolysis is carried out in a reactor provided with an atmosphere devoid of oxygen. During the process, the rubber softens, then the rubber polymers break down into smaller molecules that are exhausted from the reactor as vapors (which can be subsequently condensed to a liquid oil phase) and gases. Also formed is a carbon-containing solid residue that can further include silica, alumina, zinc oxide and/or other compounds. See, for example, U.S. Pat. No. 4,251,500A issued to Morita et al.; U.S. Pat. No. 5,264,640A issued to Platz; and U.S. Pat. No. 6,221,329B1 issued to Faulkner et al.
With advances in equipment and techniques, the main products of a modern tire pyrolysis apparatus are oil, steel (reclaimed as steel wire) and a carbon char component (“pyrolytic carbon”). Properties of pyrolytic carbon are discussed, for example, by C. J. Norris et al., in Maney Online, Vol. 43 (8), 2014, pp. 245-256, incorporated herein by reference in its entirety. Possible applications for carbon obtained by pyrolyzing waste tires are described, for instance, by C. Roy et al. in the article The vacuum pyrolysis of used tires—End-uses for oil and carbon black products, Journal of Analytical and Applied Pyrolysis, Vol. 51 pp. 201-221 (1999).